Morphology and ecological setting of the basal echinoidgenus Rhenechinus from the early Devonian of Spainand Germany

ANDREW B. SMITH, MIKE REICH, and SAMUEL ZAMORA

Smith, A.B., Reich, M., and Zamora, S. 2013. Morphology and ecological setting of the basal echinoid genus Rhenechinusfrom the early Devonian of Spain and Germany. Acta Palaeontologica Polonica 58 (4): 751–762.

Based on new material from Germany and Spain, the echinoid “Lepidocentrus” ibericus from the Early Devonian (Emsian)of northern Spain is shown to be congeneric with Rhenechinus from the Hunsrück Slate of south−western Germany. New in−formation on the lantern, pedicellariae and internal structure of the theca is provided, and confirms this genus as a member ofthe Echinocystitidae–Proterocidaridae clade and the most primitive of all Devonian echinoids. The two environmental set−tings in which Rhenechinus is found are very different: the Spanish specimens come from a relatively shallow−water bryo−zoan meadow setting while the German specimens are preserved in a deep−water setting. We deduce that the rare echinoidspecimens from the Hunsrück Slate are all allochthonous, whereas the Spanish material is preserved in situ.

Key words: Echinodermata, Echinoidea, morphology, phylogeny, Devonian, Spain, Germany.

Andrew Smith [a.smith@nhm.ac.uk] and Samuel Zamora [samuel@unizar.es], Department of Palaeontology, The Natu−ral History Museum, Cromwell Road, London SW7 5BD, UK;Mike Reich [mreich@gwdg.de], Geowissenschaftliches Zentrum der Universität Göttingen, Museum, Sammlungen &Geopark, Goldschmidtstr. 1−5, D−37077 Göttingen, Germany.

Received 11 July 2011, accepted 27 February 2012, available online 8 March 2012.

Copyright © 2013 A.B. Smith et al. This is an open−access article distributed under the terms of the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the originalauthor and source are credited.

Introduction

Although the fossil record of echinoids extends back to the Or−dovician, Palaeozoic echinoids remain scarce and very poorlyknown. This is because their skeleton is much less robust com−pared to that of their post−Palaeozoic descendants, and theyare rarely well preserved. Whereas the great majority ofpost−Palaeozoic echinoids have coronal plates that abut andare firmly sutured together, all Palaeozoic echinoids have atest constructed of imbricate plates that falls apart rapidly upondeath. Consequently, our knowledge of the early evolutionaryhistory of echinoids is patchy at best, and comes from a smallnumber of localities and horizons where sedimentary condi−tions have favoured rapid burial.

The Devonian marks an important period in the diversifi−cation of echinoids and saw the start of several lineages thatcame to dominate in the upper Palaeozoic (Kier 1965: text−fig.8; 1968). It is unfortunate, therefore, that so little is knownabout the echinoids and their ecology at this period. This stemsfrom the paucity of well−preserved echinoids that have beendescribed. Just 17 species in ten genera have been recordedfrom the Devonian, mostly from Germany or North America,and half of these are based on isolated spines, disarticulated

plates or test fragments that are so incompletely known as tobe effectively indeterminate (Smith 2011). Yet, judging fromtheir morphological disparity, echinoids appear to have beenrather diverse at this time. Of the six genera for which ade−quate material exists, three (Albertechinus Stearn, 1956, Nor−tonechinus Thomas, 1924, and Deneechinus Jackson, 1929)are members of the Archaeocidaridae, one (LepidechinoidesCooper, 1931) is a lepidesthid, another (Porechinus Dehm,1961) a palaechinid, and a third (Rhenechinus Dehm, 1953) anechinocystitid. The only Palaeozoic echinoids to have beenpreviously reported from Spain are some disarticulated platesand spines that were referred to “Archaeocidaris sp.” (Barrois1882; Sieverts−Doreck 1951), and the poorly known Lepido−centrotus ibericus (Hauser and Landeta 2007). Newly col−lected material of L. ibericus allows us to establish the trueidentity of this species and reveals its close relationship toRhenechinus hopstaetteri Dehm, 1953, from the GermanLower Devonian Hunsrück Slate. R. hopstaetteri was estab−lished on the basis of a single partial test. A second, well−pre−served individual of this species has recently come to light andwe therefore take this opportunity to provide a detailedredescription of the German species and review our under−standing of this genus and its phylogenetic position.

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Institutional abbreviations.—BSPG, Bayerische Staatssamm−lung für Paläontologie und Geologie, München, Germany;DBM, Deutsches Bergbau−Museum, Bochum, Germany;GZG, Geowissenschaftliches Zentrum, Universität Göttingen,Germany; NHM, Natural History Museum, London, UK;NM.PWL, Naturhistorisches Museum Mainz, Landessamm−lung für Naturkunde, Mainz, Germany; SMF, SenckenbergForschungsinstitut und Naturmuseum, Frankfurt/M, Germany;UO, Museum of Geology, Universidad de Oviedo, Spain.

Material and geological setting

Spanish material.—All the specimens come from the vicin−ity of Arnao village, Asturias, on the northern side of theCantabrian Zone (North Spain) within the Somiedo−Corre−cilla Unit (Fig. 1).

In this area a Lower Devonian (upper Emsian) successioncrops out in a series of old quarries between La Vela Cape and

752 ACTA PALAEONTOLOGICA POLONICA 58 (4), 2013

Undifferentiated Palaeozoic of the Cantabrian Zone

Somiedo-Correcillas

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Fig. 1. Map showing the geographical position and geological setting of the Aguión Formation and the echinoid bearing horizon (arrowed symbol), modi−fied from García−Alcalde (1992).

Arnao beach. Here the lower 60 m of the Aguión Formationare exposed and have been informally divided into three litho−stratigraphic units (sensu Álvarez Nava and Arbizu 1986); alower calcareous unit, a middle marly−shaly unit and an upperunit of green and red marls (Fig. 2). The upper unit is 24 mthick and contains a rich fauna of pelmatozoans, fenestellidbryozoans, and brachiopods. Crinoids are diverse and some−times very common, especially Trybliocrinus flatheanus andto a lesser extent Pterinocrinus decembrachiatus, Orthocrinussp., and Stamnocrinus intrastigmatus (Schmidt 1931; Breimer1962). Blastoids include Pentremitidea lusitanica and Pleuro−schisma verneuili (Johnny Waters personal communication,May 2011). The commonest brachiopod is Anathyris andthere are abundant large fenestellid bryozoan colonies (Iso−trypa sp.). Articulated specimens of Rhenechinus come from ahorizon towards the top of the upper unit (Fig. 2: arrow), al−though isolated plates are found throughout.

Argillaceous content varies within the upper unit of theAguión Formation, indicating variability in the supply ofterrigenous material. Arbizu et al. (1995) suggested this wasa major factor in controlling the different fossil assemblagesencountered within the unit. Levels where the crinoid Trybo−locrinus are common probably represent turbid palaeoenvi−ronments where there was abundant mud in suspension,whereas the level with echinoids has abundant fenestellidsand other crinoids (i.e., Pterinocrinus decembrachiatus) andappears to have been deposited in a well−oxygenated and rel−atively tranquil environment. Arbizu et al. (1995) interpretedthe entire unit as having been deposited in a typical platformenvironment, with highly variable rates of terrigenous sup−ply. The presence of marl−rich levels with well−preservedechinoderm specimens alternating with tempestite encrinitelevels suggests an offshore setting, sporadically affected bystorm events.

The bed in which the echinoids are preserved is exposed onthe foreshore at 43�34’44.6”N 5�59’02.2”W and is a firm−ground with attached crinoid holdfasts in places. Echinoids arepreserved at the base of a red clay drape that covered this sur−face and appear to have been relatively common at this level.We deduce that echinoids were living in amongst the bryo−zoan meadows in a firm−ground, level bottom community.

German material.—Both specimens of Rhenechinus hop−staetteri come from the Lower Devonian (Early Emsian)Hunsrück Slate in the Rhineland−Palatinate of south−westernGermany. The Hunsrück Slate is a Lagerstätte famous for itssoft tissue preservation (Bartels et al. 1998; Sutcliffe et al.1999). These mudrocks were deposited in a shelf−basinal envi−ronment separated by shallower swells on which a diversebenthic marine fauna thrived. Arthropods dominate, but thereare also corals, sponges, brachiopods, gastropods, cephalo−pods, and echinoderms (Mittmeyer 1980; Bartels and Brassel1990; Bartels et al. 1997, 1998; Kühl et al. 2011). Asteroids,ophiuroids and, to a lesser extent, crinoids dominate theechinoderm assemblages, but there are also rare holothuriansand two monospecific echinoid genera (Porechinus Dehm,

1961 and Rhenechinus Dehm, 1953) as well as fragments ofthe echinoid Lepidocentrus Müller, 1856 (Table 1).

Photographs and X−radiographs were taken at the NaturalHistory Museum, London.

Systematic palaeontologyPhyllum Echinodermata De Brugière, 1791(ex Klein, 1734)Stem group EchinoideaFamily Echinocystitidae Gregory, 1897Genus Rhenechinus Dehm, 1953Type species: Rhenechinus hopstaetteri Dehm, 1953, by original desig−nation; Lower Devonian (Early Emsian), Hunsrück Slate from theRhineland−Palatinate of south−western Germany.

Diagnosis.—Ambulacral zones narrow and straight; platingquadriserial throughout with every other plate a demiplateoccluded from the adradial suture. Pore−pairs uniform andwith a surrounding peripodial rim, alternately displaced toleft and right forming a biseries down the centre of eachhalf−ambulacrum. Interambulacral zones broad and com−

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SMITH ET AL.—EARLY DEVONIAN ECHINOIDS FROM SPAIN AND GERMANY 753

Fig. 2. Stratigraphical succession of the Aguión Formation at Arnao Plat−form showing lithological units, their faunal composition and types of com−munities. The level bearing echinoids is indicated by an arrow. FromArbizu et al. (1995).

posed of a large number of small, polygonal plates formingsemi−regular en chevron rows; plates imbricate. Basicoronalplate present adorally. Coronal plates with small secondarytubercles or granules only, bearing short simple spines. Teetholigolamellar.

Remarks.—Rhenechinus closely resembles the late SilurianEchinocystites, but in that taxon the interambulacral plates aremore scale−like and more irregular in arrangement, not form−ing semi−organized rows, and the outer column of primaryambulacral plates always extend to the perradius. Rhene−chinus is distinguished from all other Devonian echinoids byits quadriserial pore−pair arrangement and demiplates in theambulacra. In Albertechinus, Deneechinus, Nortonechinus,Lepidechinoides, and Porechinus the ambulacral pore−pairsare uniserial and there are no demiplates. Proterocidarids, suchas Proterocidaris and Pholidocidaris, differ from Rhenechi−nus in having enlarged oral pore−pairs and more than four col−umns of plates in their ambulacral zones.

Stratigraphic and geographic range.—Emsian, Lower De−vonian; Spain and Germany.

Rhenechinus ibericus (Hauser and Landeta, 2007)Figs. 3, 4, 5A–E.

2007 Lepidocentrus ibericus; Hauser and Landeta 2007: 66, text−figs.2, 3.

Holotype: Specimen referred to by Hauser and Landeta (2007), housedin a private collection and unnumbered in the original description.

Type horizon: Aguión Formation, Emsian, Lower Devonian.

Type locality: Cap la Vela, Arnao, Asturias, NW Spain.

Material.—Five specimens, UO DGO−23000–23003, NHMEE13984, all from the type loclity and horizon.

Diagnosis.—A species of Rhenechinus with just four col−umns of interambulacral plates in each zone. Surface ofinterambulacral plates with pitted ornament.

Description.—All specimens have collapsed post−mortembut are estimated to have been up to 45 mm in diameter.They were almost certainly globular in shape. The apicaldisc is not seen but in UO DGO−23002 there is a single geni−tal plate in isolation (Fig. 5B). This has a single gonoporeand no hydropore openings and is pentagonal in outline.Ambulacral zones are relatively narrow and composed offour columns of plates throughout. In each half column alarge primary element extends from adradial to perradialsuture and alternates with a smaller demiplate, which is ex−cluded from the adradial suture (Figs. 3B, 4B, 5A). In exter−nal view the ambulacra are flush and each element has a sin−gle rather large pore−pair with a subcircular tube−foot at−tachment rim slightly less than 1 mm in diameter. Thepore−pairs are offset on the demiplates so that there are twobiserial columns of pore−pairs in each ambulacral zone.There is usually one large mamelon (ca. 0.4 mm diameter)without a boss on the lower adradial side of the pore−pair onprimary plates, accompanied by two or three small granules(ca. 0.2 mm diameter). The occluded plates have just one ortwo granules at most. On the internal surface each primaryplate thickens perradially, giving rise to a haft that archesinwards towards the perradius forming a ridge (Fig. 4A3).This internal ridge is more strongly developed on adoralplates than on adapical plates. The perradial edge of theseplates is notched by a longitudinal groove for the radial wa−ter vessel, which therefore lay enclosed within the ambulac−ral plates. There is also a lateral canal that leads to thepore−pair on the primary ambulacral plate, and which is alsotherefore enclosed. Ambulacral elements close to the peri−stome become narrower with long adradial projections, andtheir pore−pairs become smaller (Fig. 3A). All plate edgesare bevelled, with the adradial edge of ambulacral platespassing under the adjacent interambulacral plates.

754 ACTA PALAEONTOLOGICA POLONICA 58 (4), 2013

Table 1. Echinoid specimens from the Lower Devonian Hunsrück Slate.

Museum acronymand number Original collection Species Locality Authors

BSPG 1955 I 585 leg. n. n. 1949; donated by HelmutHopstätter (Simmern), 1952

Rhenechinus hopstaetteriDehm, 1953

Quarry Kaisergrube,Gemünden

Hopstätter 1952; Dehm1953

BSPG 1960 I 164 donated by Maria Bodtländer−Gross(Bundenbach), 1960

Porechinus porosusDehm, 1961 Bundenbach Dehm 1961

GZG.INV.19996 purchased in 1963 Lepidocentrus sp. Quarry Mühlenberg nearBundenbach –

SMF.HS.426 J. Schmitt, 1966 gen. et sp. indet. Quarry Eschenbach,Bundenbach –

private collection E. Halisch (Rotenburg W.) Lepidocentrus sp. Bundenbach Beyer 1979private collection A. Seilacher (Tübingen) Lepidocentrus sp. ?Bundenbach –DBM.HS.285 – Rhenechinus hopstaetteri

Dehm, 1953Quarry Eschenbach−

Bocksberg, BundenbachBartels and Brassel 1990;Bartels et al. 1997, 1998

DBM.HS.337 – Lepidocentrus sp. Quarry Kreuzberg nearWeisel

Bartels and Brassel 1990;Bartels et al. 1998

NM.PWL.1992/234−LS – Lepidocentrus sp. Quarry Eschenbach−Bocksberg, Bundenbach –

NHM.E76446 purchased from F. Krantz in 1905 ?Rhenechinushopstaetteri

Bundenbach –

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A

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Fig. 3. Echinocystitid echinoid Rhenechinus ibericus (Hauser and Landeta, 2007). Aguión Formation, Emsian, Lower Devonian, Arnao, Asturias, NWSpain. A. UO DGO−23001 showing adoral plating and elements of the lantern. B. UO DGO−23000 showing partially articulated plating of the upper sur−face. Abbreviations: H, hemipyramid; t, tooth; 1, 2a, b, 3a–c, first three rows of interambulacral plates.

756 ACTA PALAEONTOLOGICA POLONICA 58 (4), 2013

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Interambulacral plates in the best−preserved specimenform four columns towards the ambitus (Fig. 3B). The outercolumns are composed of irregularly pentagonal plates whilethe middle two columns are composed of strictly hexagonalplates (Fig. 5A). Plates are 5–6 mm in diameter and ratherthick (0.8 mm) compared to those of R. hopstaetteri. Plateedges are bevelled with adradial faces overlapping ambulac−ral plates and interradial faces underlapping adjacent plates.Towards the apex the outer columns of plates are pinched outleaving just two interambulacral columns. At the peristomethere is a single basicoronal plate followed by two equal−sized plates, which in turn abut a large central hexagonalplate plus two adradial plates (Fig. 3A). Externally inter−ambulacral plates are ornamented by a dense circular pitting,with pits approximately 0.2 mm in diameter (Fig. 4A1). In be−tween the pits there are scattered small granules again ap−proximately 0.2 mm in diameter.

Primary spines are preserved only in the ambulacral re−gions. They are up to 2 mm in length, circular in cross−sec−tion, and without a hollow core. There is a very short base offine stereom mesh and the exterior of the shaft has fine longi−tudinal ridges of stereom. Smaller secondary spines up to 1mm in length are also present and are the only spines foundon interambulacral plates.

There are two forms of pedicellaria present; large spinatetridentate forms and short, bulbous tridentate forms (Fig. 5D,E). The large spinate tridentate forms are 1.2–1.6 mm inlength and have a broad, expanded base (0.3 mm in width)and long spine−like blades that meet for almost their entirelength and are subcircular in cross−section. Close to the baseis there a narrow gap between the blades, occupying no morethan one−third of the blade length. The interior structure andappearance of the valves is unknown. These spinate tri−dentates are found along the ambulacral zones. The secondtype of pedicellaria is similar only smaller, between 0.3 and0.6 mm in length, and with shorter, stubbier blades. Theyhave a rounded base without handles which grades into shortblades that contact along most of their length. Again there isa narrow opening, much shorter than the length of the blades,close to the base. Their interior structure is also unknown.These small pedicellariae are found in association with bothambulacral and interambulacral zones.

The lantern is preserved in two specimens, NHM EE13984and UO DGO−23001. Large hemipyramids are present but arealways partially buried under other plates of the test so thattheir overall shape is impossible to reconstruct. They taperadorally and each has a deep, well−defined outer groove for re−tractor and protractor muscle attachment (Fig. 3A). Rotulaeare of the hinge−construction type (Fig. 4A2). Both internal(Ri) and external (Re) surfaces are clearly displayed and showthe articulation facets and symmetrical ligament insertion fur−

rows for attachment to the epiphyses. Adaxially the rotula isnotched and ends as two rounded projections while proxi−mally there is a flattened condyle with a distinctive V−shapedgroove on its lower surface. Epiphyses were presumably pres−ent but are nowhere clearly seen. A flat plate seen in Fig. 4A2:e may be an epiphysis. This has only a weak lateral section un−like the axe−shaped epiphyses of crown−group echinoids illus−trated by Kroh and Smith (2010: fig. 19A–C). On one surfacethere is a long median groove, presumably to house the rotula.A single ossicle with a narrow subcircular shaft that ends in abent head with a double condyle (Fig. 4A2: c) is interpreted tobe a compass. Teeth are up to 3 mm wide with a flat axial faceand convex abaxial face. They are simple oligolamellar instructure (see Reich and Smith 2009), constructed of a biseriesof stout lath−like elements (Fig. 5B). At any one point there aresome five laths abreast on each side of the tooth.

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Fig. 4. Echinocystitid echinoid Rhenechinus ibericus (Hauser and Landeta, 2007). Aguión Formation, Emsian, Lower Devonian, Arnao, Asturias, NWSpain. A. NHM EE13984; entire specimen (A1), detail of lantern elements (A2), detail of internal haft on ambulacral plates (A3). B. UO DGO−23000, show−ing detail of ambulacral plating. Abbreviations: c, compass; e, epiphysis; H, hemipyramid; h, haft on internal face of ambulacral plate; Ri, rotula−internalface; Re, rotula−external face.

�

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1 mm

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Fig. 5. Camera lucida drawings of test plating, tooth, and pedicellariae.A–E. Rhenechinus ibericus (Hauser and Landeta, 2007), Aguión Forma−tion, Emsian, Lower Devonian, Arnao, Asturias, NW Spain. A. UO DGO−23000, adapical ambulacral and interambulacral (shaded) plating. B. UODGO−23002, genital plate. C. UO DGO−23001, tip of oligolamellar toothseen in axial view. D. UO DGO−23001, spinate tridactylous pedicellaria.E. UO DGO−23000, two small tridactylous pedicellariae (E1 and E2).F. Rhenechinus hopstaetteri Dehm, 1953, DBM.HS.285, spinate tridacty−lous pedicellaria with basal element.

Remarks.—This species differs from Rhenechinus hopstaet−teri in having fewer more regularly polygonal interambulacralplates with rather thicker and more upright sutures. Further−more, the surface of interambulacral plates shows a distinctivepattern of fine circular pits towards their centre, which is neverseen in R. hopstaetteri.

Stratigraphic and geographic range.—Emsian, Lower De−vonian; Cap la Vela (Arnao), northern Spain.

Rhenechinus hopstaetteri Dehm, 1953Figs. 6, 7.

1952 “Ein Seeigel aus dem Hunsrückschiefer”; Hopstätter 1952: 33.1953 Rhenechinus hopstätteri; Dehm 1953: 93, pl. 5: 1–4.1961 Rhenechinus hopstätteri Dehm, 1953; Kuhn 1961: 33, figs. 15, 16.1966 Rhenechinus hopstatteri Dehm, 1953; Kier 1966: U303, fig. 224.2.1970 Rhenechinus hopstätteri Dehm, 1953; Kutscher 1970a: 40.1970 Rhenechinus hopstätteri Dehm, 1953; Kutscher 1970b: 96.1980 Rhenechinus hopstaetteri Dehm, 1953; Mittmeyer 1980: 38.1990 Rhenechinus hopstätteri Dehm, 1953; Bartels and Brassel 1990:

181, fig. 169.1997 “Seeigel Rhenechinus hopstätteri mit erhaltenen Stacheln“; Bar−

tels et al. 1997: 49, fig. 61.1998 Rhenechinus hopstaetteri Dehm, 1953; Bartels et al. 1998: 210,

fig. 188.1999 Rhenechinus hopstätteri Dehm, 1953; Jahnke and Bartels 1999: 43.2000 Rhenechinus hopstätteri Dehm, 1953; Jahnke and Bartels 2000: 43.Holotype: BSPG. 1955 I 585.Type horizon: Hunsrück Slate, Lower Emsian, Lower Devonian.Type locality: Bundenbach, Rhineland−Palatinate, Germany.

Material.—One specimen, DBM.HS.285, Eschenbach−Bocks−berg mine, Bundenbach, Rhineland−Palatinate, Germany.

Diagnosis.—A species of Rhenechinus with up to 8 inter−ambulacral columns in a zone with plates of rather irregularshape and size; surface of plates lacking pitted ornamentation.

Description.—The holotype (Fig. 6) shows an articulatedtest in oral view and the new specimen (Fig. 7) is a completetest squashed in lateral profile. The test must have beensubglobular in life and taller than wide, with a diameter up toapproximately 10 cm. Apical disc unknown but relativelysmall, as the ambulacra converge adapically (Fig. 7A).

Ambulacral zones are narrow and biserial with a primaryelement alternating with a small demiplate in each half ambu−lacrum (Fig. 6B). Pore−pairs are prominent and offset forminga double series in each column. The pore−pairs are about 1 mmin width with an obvious periporal rim; the inner pore piercesthe plate rather than forming a marginal notch. There are smallgranules scattered on the adradial and perradial sides of thezone of pore−pairs. Towards the peristome plates become nar−rower and wider, and pore−pairs become smaller. Ambulacralplating appears to extend further onto the peristome thaninterambulacral plates (Fig. 6C).

Interambulacral plates are up to 6 mm in width and irregu−larly polygonal in outline. At the widest point there are somenine plates abreast. Only the adradial series of plates form aregular column and these may bear slightly larger tuberclesthan other plates. Plates are relatively thin—not much morethan 0.6 mm in thickness, and have bevelled edges. A single

plate forms the adoral boundary to the peristome and this isfollowed by two plates that just touch interradially and then bythree plates, a large central hexagonal plate and two adradialpentagonal plates (Fig. 6C).

A membrane of small platelets covers the peristome (Fig.6C), which is about 20 mm in diameter. While ambulacralplates extend a little way onto this membranous region, theyare confined to the outer region.

Spines are up to 4 mm in length and are concentrated in theperistomial and ambulacral zones (Figs. 6A, 7B). There areseveral slender spines to each ambulacral plate, with some−what longer spines being found adradially and shorter spinesperradially. Small spines are also found on interambulacralplates but are not nearly as dense nor as long. Small spinatetridactylous pedicellariae are present on interambulacralplates (Figs. 5E, 7B). These are approximately 1 mm long andhave a swollen base and long spine−like blades that are in con−tact along most of their length. The pedicellarial head rests ona small element some 0.2 mm in width that is either a stem ele−ment (as described by Haude 1998) or a small tubercle (preser−vation is not adequate to distinguish).

The lantern is present but internal. Hemipyramid tips can beseen (Fig. 7C) and show a deep abaxial groove for retractor andprotractor muscle insertion. The teeth that protrude have a flataxial face and are simple oligolamellar in strucure with four orfive laths abreast on each half. They end at a simple point.

Remarks.—The original description given by Dehm (1953)is detailed and largely correct. The new specimen providesadditional information about the test shape and about the dis−tribution of spines and pedicellariae. This is a species that at−tains almost twice the size of the Spanish species but we donot think the diagnostic differences are simply size related.Although R. ibericus may increase the number of inter−ambulacral columns in each zone as it grows, the surface or−nament in the two species is quite distinct with the Spanishspecies having a strongly pitted surface and the German spe−cies a smooth, unpitted surface.

One other echinoid, Porechinus porosus is known from thesame formation (Dehm 1961) and this does have a stronglypitted ornament to its plate surfaces. However, it differs inhaving distinctly thicker plating and ambulacral plates that areuniserially arranged. All the other echinoid specimens of theLower Emsian Hunsrück Slate (Lepidocentrus spp.; Table 1)are based on fragmentary material and need to be revised. Thisis made especially difficult by the incomplete nature of thetype material of Lepidocentrus (Müller 1857).

Stratigraphic and geographic range.—Hunsrück Slate, LowerEmsian, Lower Devonian; Gemünden and Bundenbach, Rhine−land−Palatinate, Germany.

Discussion and conclusionsRhenechinus was originally treated as a lepidocentrid byDehm (1953) and Hauser and Landeta (2007) also assigned

758 ACTA PALAEONTOLOGICA POLONICA 58 (4), 2013

their Spanish specimen to Lepidocentrus. Kier (1965: 450,1966) transferred Rhenechinus to the Echinocystidae on the

basis of its plesiomorphic similarity to Echinocystites. How−ever, much of its anatomy was at that time incompletely

http://dx.doi.org/10.4202/app.2011.0098

SMITH ET AL.—EARLY DEVONIAN ECHINOIDS FROM SPAIN AND GERMANY 759

A

B

C

5 mm

3a

3c2a

1

2b

ps

t

5 mm2 mm

3b

Fig. 6. Echinocystitid echinoid Rhenechinus hopstaetteri Dehm, 1953, BSPG.1955 I 585 (holotype), Hunsrück Slate, Lower Emsian, Lower Devonian;Gemünden, Rhineland−Palatinate, Germany. A. General view. B. Detail of ambulacral plating. C. Detail of oral region. Abbreviations: ps, peristomialmembrane; t, tooth; 1, 2a, b, 3a–c, first three rows of interambulacral plates.

known. The new specimens from Spain (Figs. 3–5) and Ger−many (Fig. 7) provide important new information on themorphology of Rhenechinus and help confirm its phylogen−etic position as lying between Echinocystites and the Pro−terocidaridae. Based on our new material we now confirmthat Rhenechinus retains many primitive features comparedto other Devonian echinoids. Firstly, as first noted by Jesio−nek−Szymańska (1982), its teeth have a very primitive con−

struction−termed simple oligolamellar (Reich and Smith2009), like those of Echinocystites but in marked distinctionto the teeth of archaeocidarids, lepidesthids, lepidocentrids,and palaechinids, all of which have true lamellar teeth. Sec−ondly, the radial water vessel in Rhenechinus is enclosedwithin the ambulacral plates, at least adorally. Each primaryambulacral plate has a large internal haft that projects per−radially to underlie the radial water vessel (Fig. 4A3), again a

760 ACTA PALAEONTOLOGICA POLONICA 58 (4), 2013

A

BC

ambamb

5 mm

amb

ap

pst

amb

p

ps

p

2 mm 2 mm

Fig. 7. Echinocystitid echinoid Rhenechinus hopstaetteri Dehm, 1953, DBM.HS 285, Hunsrück Slate, Lower Emsian, Lower Devonian; Eschenbach−Bocksberg mine, Bundenbach, Rhineland−Palatinate, Germany. A. General view. B. Detail of adoral plating showing pedicellariae and spines. C. Detail ofadoral ambulacral plating. Abbreviations: amb, ambulacral zone; ap, apical disc region; p, pedicellaria; ps, pedicellarial stalk element; pst, peristome.

feature seen in all Ordovician and Silurian echinoids, includ−ing Echinocystites, but in none of the other Devonian formsexcept Albertechinus. Thirdly, the lack of enlarged primarytubercles and spines distinguishes Rhenechinus from otherDevonian echinoids with the exception of Porechinus.Larger tubercles and spines are wanting from all Silurian andOrdovician echinoids and it is only in the Devonian thatechinoids with larger articulated spines are first encountered.Finally, the ambulacral plating in Rhenechinus is identical tothat seen in Echinocystites, and consists of a primary plate al−ternating with a demiplate. This pattern of plating is notfound in any other Devonian echinoid. All these featuresconfirm the primitive status of Rhenechinus and its closesimilarity to Echinocystites. However, compared to Echino−cystites, Rhenechinus has more regularly organized inter−ambulacral plating and distinct peripodial rims around itspore−pairs, as in proterocidarids. Rhenechinus is thereforephylogenetically intermediate between Echinocystites andthe Proterocidaridae.

Rhenechinus provides further information on the types ofpedicellariae present in Palaeozoic echinoids. It possessedonly tridactylous pedicellariae, although these were of twoforms, large and small. The large spinate pedicellariae aresimilar in appearance to those that have been found in otherDevonian echinoids (Lepidocentrus; Haude 1998). They arethree−bladed and rest either on a small basal element or di−rectly onto a tubercle. Ophicephalous and globiferous pedi−cellariae, which have been reported from some Carbonifer−ous taxa (Geis 1936; Coppard et al. 2012), are wanting. Thissupports the view that ophicephalous and globiferous pedi−cellariae evolved only in archaeocidarids. Larger pedicel−lariae are more common along the ambulacra suggesting thattheir primary role may have been to protect the tube−feetfrom pests and parasites. Smaller pedicellariae are foundscattered over interambulacral plates and may have beenrather common, although they are now preserved only insmall patches.

Our new material also provides information on the struc−ture of Palaeozoic echinoid lanterns. Specifically we recog−nize for the first time that compass elements were present(Fig. 4A2). These slender elements act to regulate the volumeof the peripharyngeal coelom as the lantern moves in and outof the test during feeding. Although lanterns have been de−scribed for a small number of Ordovician and Silurian echi−noids (Aulechinus, Palaeodiscus, Echinocystites, and Apti−lechinus) none have preserved compasses. Compasses areslender and rather fragile elements by comparison to theother elements that make up the lantern of sea urchins, sotheir absence may be taphonomic rather than genuine. Theirdefinitive presence in Rhenechinus suggests that they mustalso have been present in many of the other Palaeozoicechinoids by phylogenetic implication.

It is unusual to find the same echinoid genus in two suchvery different environmental settings and this requires com−ment. The main difference between the Spanish and Germandeposits is that the former were formed under normal shal−

low marine conditions on a terrigeonous−carbonate ramp(Arbizu et al. 1995; see above), while the Hunsrück Slatelacks carbonates and was deposited under deeper−water,shelf−basin conditions (Bartels et al. 1998). The fact that theechinoids are common, and preserved so well at Arnao, indi−cates they are autochthonous. By contrast, after hundreds ofyears of work on the fauna of the Hunsrück Slate there areonly two definite specimens of Rhenechinus known. Be−cause echinoids preserved in the Hunsrück Slate are so rare(around ten specimens only are known; Table 1) they are pre−sumably allochthonous, washed into the basins from shal−lower habitats in nearby local swells.

AcknowledgementsMR is grateful to Christoph Bartels (Deutsches BergbaumuseumBochum, Germany), Herbert Lutz (Naturhistorisches Museum Mainz,Germany), Martin Nose (Bayerische Staatssammlung für Paläonto−logie und Geologie München, Germany), and Eberhard Schindler(Senckenberg Forschungsinstitut und Naturmuseum Frankfurt/M.,Germany), who kindly allowed us to study Hunsrück specimens in theircare. MR also thanks Reimund Haude (Göttingen, Germany) and PeterHohenstein (Lautertal, Germany) for fruitful discussions on HunsrückSlate echinoderms. This study was supported in part by a Synthesysgrant (http://www.synthesys.info/), a programme financed by Euro−pean Community Research Infrastructure Action under the FP6 “Struc−turing the European Research Area Programme” to MR (GB−TAF−2446). We would like to thank Miguel Arbizu and Isabel Méndez−Bedia (both Oviedo University, Spain) for providing work facilities inArnao and Jenaro García−Alcalde for some geological comments onArnao. Ms. Isabel Pérez (University of Zaragoza, Spain), produced ourFig. 1. Johnny and Will Waters (Appalachian State University, USA)provided invaluable help during the field work. SZ acknowledges apostdoctoral fellowship from the Ministerio de Educación of Spain.

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